Growth of crystals



Aug. 2, 1960 D. c. REYNOLDS ET AL 2,947,613,

GROWTH oF CRYSTALS Filed March 16, 1956 www.,

QAM/M "1. VW'- nited States Patented Aug. 2, 1960 hcc 2,941,613 GROWTH or CRYSTALS Donald .C. Reynolds, Springtield, hio, and` Stanley I. Czyzak, Royal Oaks, Mich., assignors to the United States of America as represented by the Secretary of the Air Force FiledMar. 16, 1956, Ser. No. 572,170

'3 Claims. (Cl. 23-294) (Granted under Title 35, U.S. Code (1952), sec. 266) The invention described herein may be manufactured and' used by or for the United States Government for governmental purposes without the payment tous of any royalties thereon.

This invention relates to a means and a method for accomplishing crystal growth.

Asv a background for making the present invention as it is presented and claimed herein clearly understandable, a chemical element or a chemical compound has an energy content which commonly causes it to exist in three states. At low temperatures it may be solid, with the addition of heat it may liquefy and with the addition of more heat it may become vapor. Some materials sublime from the solid state directly to the gaseous phase under favorable pressure conditions'. In an environment of unimpeded orientation an element or a compound may arrange itself in a crystalline form with a space lattice characteristic of that element or' compound'.

A general statement of the exact nature and of the subtance or the subject matter of the present invention consistent with the claims which are appended hereto and together with the operation and the purpose of the in' vention, is that the exact nature or the essential character or constitution of the present invention is the production in the held of solid State physics of a crystal, such as cadmium suliide, zinc sullide and their equivalents. The substance or the subject matter of the present invention is a method for making a crystal of a desired size and composition by its solidication from its gaseous state and a `furnace comprising a plurality of temperature zones within which a closed bottom quartz cup receives the crystal as a solid from a retort tube orifice. The retort tube opens into the upper part of a quartz retort or tank containing the material from which a crystal is to be made. The material is in both its solid and gaseous states within the retort. The retort or tank is supported from` the top of the quartz cup. An atmosphere of hydrogen sulde at an illustrative pressure of from a onefourth to a one-half atmosphere is maintained within the quartz cup within which the crystal forms.

A general object of the present invention is to provide a new and an improved method for making a crystal of a desired size by a vaporization process.

A further object is to provide a new and an improved I furnace for use in the field of solid state physics and the like.

Another object is to provide a crystal of cadmium sulfide, zinc sulde or their equivalents.

In the accompanying drawing:

Fig. l is an elevational View partly in section, partly broken away and partly diagrammatic of a furnace embodying the present invention;

Fig. 2 is a plan view from above taken about from the line 2-2 of Fig. 1;and

Fig. 3 is a perspective enlarged View of a crystal made in the furnace shown in Fig. l, in the practice of the method of the present invention.

The furnace shown in Fig. l of the accompanying drawing comprises illustratively an upwardly opening quartz cup 1`, or hollow cylindrical tube with a closed bottom, which is positioned preferably within a multiple winding or a multiple heating element furnace. The furnace illustratively may comprise a rst pair of windings 2 and 2', each of which is maintained at a temperature which is controlled by its own` thermocouple 3 or 4 from a temperature controll device 5, and a second winding 6, which is maintained at a temperature by operation of a thermocouple 7 and another temperature control' device 8; The first pair of windings 2 and 2 illustratively may maintain a temperature slightly below the sublimation temperatureV of cadmium sulfide, whereas the third winding 6 maintains a temperature slightly above the sublimation temperature of the solid to be vaporized. The vfurnace is provided in the usual manner with an insulant 9` against objectionable heat lossV with the insulant positioned outwardly of the windings 2 and` 6.

A second. upwardly opening quartz cup 10, which is a hollow cylinder with a closed bottom, is mounted to oat axially and adjustably within the cup 1. Illustratively a soft iron ring, or a plurality of soft iron slugs 11, 11', etc., may be secured to the oating cup 10 of a desired size by screws 12, 12', etc. within a suitable magnetic field, such as that maintained by a pair of horseshoe magnets 13` and 1:4 with their like poles joined, as by being welded' together or the like. The magnets I3 and 14 carry pairs of` bosses 13",V 13 and 14', 14 which are tapped and threaded to receive securing screws 15, 15', etc. which attach the horseshoe magnets to end members 16 and 17 through which yoke supporting screws 18' and 19" thread, respectively, throughout, The screws 1S and 19 are rotated simultaneously to raise or to lower the horseshoe yoke by operation ofv a gear train within a gear box 20". The gear train is actuated through a shaft 21 by a reversible motor 22 which may be energized from a potential source 23 upon the closing of a switch Z4.

A retort 25, or tank, is positioned immovably within the floating cup 1d'. In the assembly shown, the retort 25 is supported byr a cap 26 which makes substantially a gas` tight seal. with the upper edge of the outer cup 1 by means of a ground glass joint 27 or the like. Suitable means, such as a stop cock or valve 33 is provided for evacuating and then` introducing hydrogen sulfide gas into the interior of the floating `cup 10; The retort 25 may be charged with a material from which a crystal of a desired size is to be made, such as cadmium sulde, zinc sulfide or an equivalent, preferably in powdered solid state. The retort may then be sealed by the use of heat and `connected through a rod portion 28 with the cap 26 and installed in place.

The retort 25 is positioned to exhaust downwardly centrally of the bottom of the floating cup l0, as through a tube 29 or the like. The attached end 3d of the tube 29 opens into and receives its gaseous input from the.

upper part of the retort 25, as represented in the drawing. The orice 31 in the unattached end of the tube 29, preferably is disposed centrally of the bottom of the lloating cup 10.

The proximity ofthe output orifice 31 in the unattached end of the tube 29, to the inner surface of the bottom of the floating cup 1i) is closely adjustable. This adjustment is accomplished by operation ofthe reversible motor 22 which rotates the screws 18 and 19 to adjust the yoke carrying the magnets 13 and 14. The iron slugs 12 and 12 follow the position of the magnets 13 and 14 and carry the oating cup 1).

An illustrative cadmium sulfide solid crystal 32 made by the disclosed method with the equipment shown in Figs. l and 2 of the drawing is represented in enlarged dimensions in Fig. 3 of the drawing.

A preferred method by which the crystals contemplated hereby are made comprises the use of the above described equipment. Prior to the assembly of the tank or retort 25 with the stern 28 of the cap 26, a desired quantity of solid, granular cadmium sulfide, or other material from which a crystal is to be made and of a desired degree of purity, is charged into the tank or retort 25 through its neck opening. The neck opening of the retort 25 is then closed by union with the cap stem 28, using heat to accomplish the seal. The assembly may be evacuated then charged with hydrogen sulide through the stop cock 33, or the charged retort 2.5, together with cups 1 and 10 unassembled, may be placed in an outside container, not shown, where they may be evacuated, charged, where a sulfide crystal is in process of manufacture, with hydrogen sulfide gas of about l5 inches of mercury pressure and in that atmosphere the capsule 25 is inserted into the tube or cup 10 and the cap 26y caused to make a gas-tight union with the open top of the outer quartz cup 1. The cup 1, sealed by the cap 26, is then inserted through the yoke of the magnets 13 and 14 and is positioned within the furnace, consisting of the windings 2 and 6 or of comparable construction. The retort discharge orifice 31 is spaced a desired distance from the inner surface of the bottom of the cup 1t), as determined by the position of the magnet yoke controlled by the pair of screws 18 and 19 and the motor 22.

The furnace is then brought to its operating temperature, with the first windings 2 and 2 slightly below the sublimation temperature of cadmium sulfide, or other desired material, and the second winding 6 slightly above the material sublimation temperature, which for cadmium sulfide is in the neighborhood of 1000 C. The .furnace temperature will vary with both the material composition and with its degree of purity. Experimentally crystals have been grown in from 24 to 168 hours. The furnace illustratively may be maintained at its operating temperature for about a week or more in the building of crystals of usable size during which time the separation between the retort discharge orifice 31 and the bottom of the cup may be increased to accommodate the increasing size of the growing crystal.

The crystals of cadmium sulfide grown by this disclosed method have been of a general hexagonal prismatic shape. Cadmium sulfide is an excellent photoconductor. Chemically pure cadmium sulfide has only one peak occurring at the absorption cutoff, with very rapid rise and decay of the photocurrent on either side of the peak.

It is to be understood that the method and the apparatus which are disclosed herein have been submitted for the purposes of explaining and illustrating an operative embodiment of the present invention and that similarly functioning method steps and apparatus arrangements may be made without departing from the spirit of `the present invention.

That we claim is:

1. The method of growing a single crystal of uniform lattic structure throughout embodying the step of discharging a material selected from the group of cadmium sulfide and zinc sulfide in a gaseous state from an orifice toward a surface movable away from the orifice as the crystal increases in thickness under temperature conditions controlling the orientation of atomic material into a single uniform crystal lattice.

2. A method of growing a single crystal of uniform lattice structure throughout and of a prescribed composition selected from the group of cadmium sulde and zinc sulfide by following the steps of imparting one zone of heat to the composition to convert it to the gaseous state, conducting the gaseous composition to an orifice, discharging the gaseous composition toward a surface in a hydrogen sulfide atmosphere at a second zone of heat at a temperature slightly below the composition sublimation of the atoms of the composition temperature of 1000 C. for cadmium sulfide and controlling the orientation of the atoms of the composition in a single crystal of uniform lattice structure throughout, moving the surface away from the orifice as the crystal increases in thickness without change in the uniformity of the crystal lattice structure being formed, and maintaining.

the crystal growth conditions continuously for a prescribed period of time and size of crystal growth.

3. A crystal growing process of enclosing solid cadmium sulfide within a container having an outlet; applying a melting point heat level to the solid cadmium sulfide to change its physical solid state into its physical vapor state with suicient attendant increase in volume to cause the vaporous cadmium sulfide to pass out of the container outlet into a quartz cup hydrogen sulfide atmosphere maintained at a pressure of from one-fourth to one-half atmosphere and at a temperature slightly below the sublimation temperature of cadmium sulfide at lwhich optimum conditions exist for the continuous growth `of a single cadmium sullde crystal in its atomic orientation lattice.

References Cited in the file of this patent UNITED STATES PATENTS v:873,812 Walter Dec. 17, 1907 1,284,787 Sartako Nov. 12, 1918 1,560,926 Wilkinson Nov. 10, 1925 1,758,741 Gaskill May 13, 1930 1,941,610 Macready Jan. 2, 1934 2,813,811 Sears Nov. 19, 1957 2,843,914 Koury July 22, 1958 2,890,939 Ravich June 16, 1959 OTHER REFERENCES Buckly: Crystal Growth, 1951, page 97.

Mack et al.: Textbook of Chemistry, Ginn and Co., copyright 1949, p. 267.

anni. 

2. A METHOD OF GROWING A SINGLE CRYSTAL OF UNIFORM LATTICE STRUCTURE THROUGHOUT AND OF A PRESCRIBED COMPOSITION SELECTED FROM THE GROUP OF CADMIUM SULFIDE AND ZINC SULFIDE BY FOLLOWING THE STEPS OF IMPARTING ONE ZONE OF HEAT TO THE COMPOSITION TO CONVERT IT TO THE GASEOUS STATE, CONDUCTING THE GASEOUS COMPOSITION TO AN ORIFICE, DISCHARGING THE GASEOUS COMPOSITION TOWARD A SURFACE IN A HYDROGEN SULFIDE ATMOSPHERE AT A SECOND ZONE OF HEAT AT A TEMPERARATURE SLIGHTLY BELOW THE COMPOSITION SUBLIMATION OF THE ATOMS OF THE COMPOSITION TEMPERATURE OF 1000*C. FOR CADMIUM SULFIDE AND CONTROLLING THE ORIENTATION OF THE ATOMS OF THE COMPOSITION IN A SINGLE CRYSTAL OF UNIFORM LATTICE STRUCTURE THROUGHOUT, MOVING THE SURFACE AWAY FROM THE ORIFICE AS THE CRYSTAL INCREASES IN THICKNESS WITHOUT CHANGE IN THE UNIFORMITY OF THE CRYSTAL LATTICE STRUCTURE BEING FORMED, AN MAINTAINING THE CRYSTAL GROWTH CONDITIONS CONTINUOUSLY FOR A PRESCRIBED PERIOD OF TIME AND SIZE OF CRYSTAL GROWTH. 